Endurance exercise performance is known to deteriorate as the surrounding ambient temperature increases (Galloway S. D. and Maughan R. J., Effects of Ambient Temperature on the Capacity to Perform Prolonged Cycle Exercise in Man, Med. Sci. Sports Exerc. 1997: 29: 1240-1249), which is further exacerbated when combined with increasing humidity. Watson P., Otani H., and Maughan R. J., Influence of Relative Humidity on Prolonged Exercise Capacity in a Warm Environment, British Journal of Sports Medicine 2011: 45: A3-A4. As a result, there appears to be a strong link between increases in thermoregulatory strain due to elevations in both metabolic and ambient heat, which may result in impaired endurance performance.
In particular, studies have proposed that endurance performance in hotter climates is limited by attainment of a critical core body temperature of approximately 40° C. Gonzalez-Alonso J., Teller C., Andersen S. L., Jensen F. B., Hyldig T., and Nielsen B., Influence of Body Temperature on the Development of Fatigue during Prolonged Exercise in the Heat, J. Appl. Physiol. 1999: 86: 1032-1039; Nielsen B., Hales J. R., Strange S., Christensen N. J., Warberg J., and Saltin B., Human Circulatory and Thermoregulatory Adaptations with Heat Acclimation and Exercise in a Hot, Dry Environment, J. Physiol. 1993: 460: 467-485. The body may use this critical core temperature as guide for setting judgment alteration and effort perception in an attempt to complete a given task as quickly as possible without achieving a dangerously high core temperature. Marino F. E., Anticipatory Regulation and Avoidance of Catastrophe during Exercise-Induced Hyperthermia, Comp. Biochem. Physiol. B. Biochem. Mol. Biol. 2004: 139: 561-569.
In addition to core temperature values, skin temperature in response to exercise has also been identified as a factor that the body uses to regulate endurance performance in hotter environments. Kenefick R. W., Cheuvront S. N., Palombo L. J., Ely B. R., and Sawka M. N., Skin Temperature Modifies the Impact of Hypohydration on Aerobic Performance, Journal of Applied Physiology 2010: 109: 79-86; Sawka M. N., Cheuvront S. N., and Kenefick R. W., High Skin Temperature and Hypohydration Impair Aerobic Performance, Experimental Physiology 2012: 97: 327-332. For example, when the environmental temperature is high, fatigue may set in when the skin temperature exceeds 35° C., even though the core temperature is less than 40° C. Latzka W. A., Sawka M. N., Montain S. J., Skrinar G. S., Fielding R. A., Matott R. P., and Pandolf K. B., Hyperhydration: Tolerance and Cardiovascular Effects during Uncompensable Exercise-Heat Stress, J. Appl. Physiol. 1998: 84: 1858-1864; Montain S. J., Sawka M. N., Cadarette B. S., Quigley M. D., and McKay J. M., Physiological Tolerance to Uncompensable Heat Stress: Effects of Exercise Intensity, Protective Clothing, and Climate, J. Appl. Physiol. 1994: 77: 216-222.
Various efforts have been made to extend endurance performance by extending the time before the body reaches an excessive core temperature and/or skin temperature. One of the most widely adopted practices is that of pre-cooling. Pre-cooling can be applied externally using a variety of methods such as cold air exposure, cold water immersion, ice vests, water perfused suits, phase change materials, and forearm cooling, internally via the use of cold drinks, or via combinations of these methods. The methods are geared toward achieving reduced body heat content prior to exercising and increasing the body's ability to store endogenous and exogenous heat and improve exercise performance.
While it has been found that external pre-cooling does achieve reduced body heat content and has a beneficial effect on endurance performance, it has also been found that certain applications can have an adverse effect. Specifically, pre-cooling applications that are too cold and/or are applied for too long can trigger the body's defensive mechanisms against falling body temperatures, causing the body to avoid or resist lowering the core temperature.
Furthermore, it is well known that different anatomical parts of the body experience differing rates of heat loss. U.S. Pat. No. 7,089,995 to Koscheyev et al. For example, discrete areas of the body having a higher efficiency in transferring heat to or from the body typically have a higher density of cells making up the body tissue along with a higher amount of vascularization in the body tissue. Such regions include area with little subcutaneous fat deposition, and a high density of blood vessels close to the skin surface. Examples include forearm, wrist, lateral thoracic area (rib cage), upper torso, paraspinal area, occipital and parietal head areas, gluteal and medial or inner thigh, shoulder, pectoral region, ankle, and groin area. Because of the greater heat transfer efficiency, these regions may be targeted for more effective control of body temperature during exposure to environmental extremes. Based on these differences among discrete regions of the body, pre-cooling may be ineffective if the pre-cooling application is applied to areas with lower heat transfer rates and/or applied too broadly so that energy is wasted by cooling areas where the cooling effect is not transferred as effectively.
As an example, U.S. Pat. No. 8,585,746 describes a vest that covers a torso area of an individual having chambers 30, 40 on the front and back of the vest that are filled with water (or ice when frozen). Because the chambers 30, 40 cover the entire front and back of the torso areas, the cooling areas of the vest are positioned adjacent a large surface area of the body, which does not target specific anatomical regions of the body having more efficient heat transfer rates. In body areas where vital organs are located, the use of ice cooling might actually be counter-effective in reducing body heat content. In addition, superficial muscles used during movement should be avoided as ice cooling moves muscle temperature away from its optimum. Furthermore, these chambers 30, 40 are sized to hold up to approximately a gallon of water, which can add approximately 3-4 kilograms to the weight of the vest when fully activated with water and after being placed in the freezer for the indicated amount of time. The additional weight of the vest makes it more difficult to wear the vest for long periods of time, and may counteract any beneficial results achieved by lowering the body heat content.
As a result, it may be desirable to provide a garment that is designed to provide pre-cooling to specific areas of the body having greater heat transfer efficiency, which uses a light-weight cooling material that reduces body heat content without adverse effects.